Typical projection systems usually include a source of light, illumination optics, one or more image-forming devices, projection optics and a projection screen. The illumination optics collect light from one or more light sources and direct that light in a predetermined manner to one or more image-forming devices. The image-forming devices, controlled by an electronically conditioned and processed digital video signal or by other input data, produce images corresponding to the video signal or to that data. Projection optics then magnify the image and project it onto the projection screen. White light sources, such as arc lamps, in conjunction with color-maintaining systems, have been and still are predominantly used as light sources for projection display systems. However, recently, light emitting diodes (LEDs) were introduced as an alternative. Some advantages of LED light sources include longer lifetime, higher efficiency and superior thermal characteristics.
One example of an image-forming device frequently used in projection systems is a digital micro-mirror device, or digital light processing device (DLP). The main feature of a DLP is an array of tiltable micro-mirrors. The tilt of each mirror is independently controlled by the data loaded into a memory cell associated with each mirror, so that the mirrors steer reflected light and spatially map a pixel of video data onto a pixel on a projection screen. Light reflected by a mirror in an ON state passes through the projection optics and is projected onto the screen to create a bright field. On the other hand, light reflected by a mirror in an OFF state misses the projection optics, which results in a dark field. A color image may be produced with a single DLP by color sequencing, or, alternatively, with three DLPs, each illuminated with a primary color.
Other examples of image-forming devices include liquid crystal panels, such as a liquid crystal on silicon device (LCoS). In liquid crystal panels, the alignment of the liquid crystal material is controlled incrementally (pixel-to-pixel), as determined by the data corresponding to a video signal. Depending on the alignment of the liquid crystal material, polarization of the incident light may be altered by the liquid crystal structure. Thus, with the appropriate use of polarizers or polarizing beam splitters, dark and light regions corresponding to the input video data may be created. Color images are formed using liquid crystal panels in a manner similar to DLPs, by using a sequential color approach with one LCoS device or by using a separate LCoS device for each primary color.
Another type of an image-forming device is a high temperature polysilicon liquid crystal device (HTPS-LCD). HTPS-LCD also includes a liquid crystal layer, in which the alignment can be controlled incrementally (pixel-to-pixel), as determined by the data corresponding to a video signal. The liquid crystal layer is sandwiched between a glass substrate and an array of transparent electrodes, thus being adapted for operation in transmission. At the corner of each HTPS-LCD pixel, there is a microscopic thin film transistor.